The dynamic range of a communication system is often limited by the nonlinear characteristics of devices that are used in the system. Harmonics, or distortion products, generated by the nonlinear devices interfere with transmission of the desired signal. Usually, second-order distortion products are most troublesome because they are most likely to fall within the bandwidth of the communication system or may fall within adjacent bands in a multichannel system. An example of such a nonlinear device is a directly modulated laser diode that is frequently used in optical communication systems. The laser diode generates a light beam which is modulated by applying an information carrying signal to the laser diode. The modulation signal may include one or a number of channels.
Predistortion linearization is a known technique wherein a predistortion circuit containing an additional nonlinear device compensates for the characteristics of the nonlinear communication device. The predistortion circuit generates distortion products which are approximately equal in amplitude but opposite in phase to the distortion products generated by the nonlinear communication device. In the overall transfer characteristic between the input of the predistortion circuit and the output of the nonlinear device, second order distortion products are reduced or eliminated.
It is desirable to provide communication systems having a large dynamic range. This is particularly difficult in wideband optical communication systems which may have a bandwidth of up to several gigahertz and transmit multiple channels simultaneously.
It is a general object of the present invention to provide a second order predistortion linearization circuit for linearizing a nonlinear device.
It is another object of the present invention to provide communication systems having large dynamic ranges.
It is a further object of the present invention to provide a circuit for linearizing the characteristics of a laser diode.
It is a further object of the present invention to provide improved optical communication systems.
It is yet another object of the present invention to provide second order predistortion linearization circuits which are simple in construction and low in cost.